EP 1 709 667 discloses a metal halide lamp which is made of translucent ceramic. It is surrounded by an outer tube. The vessel has a given length L of a space between the electrodes and a given internal diameter D. The ratio L/D, the so-called aspect ratio, is in the range 4.0≦L/D≦10.0.
In other words the vessel has a high aspect ratio L/D. A main part of said vessel extends between two electrodes.
A typical ratio R/r for such lamps is in the range 3.4≦R/r≦7.0. The parameter R is an average internal diameter of the portion of the outer tube positionally corresponding to the space between the electrodes, the so-called main part. The parameter r is an external diameter of the main part, within a region positionally corresponding to, in a radial direction of the outer tube and the discharge vessel, the space between the electrodes, on a cross-sectional surface where an outer circumference of the arc tube comes closest to an inner circumference of the outer tube.
WO 2010/018048 discloses a microwave lamp with a vessel whose diameter is reduced towards the ends.